Oxidative modification of proteins has been implicated in aging and age- related disorders. However, the contributions of various kinds of reactive oxygen species in the age-related processes has not been rigorously examined. In an effort to determine the mechanism of protein oxidation by alkyl-, peroxy-, and/or alkoxy-radical-mediated processes, the oxidation of a model peptide, melittin, by azo-bis-2-amidinopropane HCl (AAPH) has been studied. AAPH decomposes in aqueous solution to generate alkyl-radicals which, in the presence of oxygen, yield peroxy- radicals and alkylperoxides that in the presence of transition metals are converted to the more reactive alkoxy-radicals. Results of earlier studies demonstrated that histidine, tyrosine, methionine, and tryptophan residues of E. coli glutamine synthetase are targets for AAPH-mediated radical damage. Because the 26-amino acid peptide, melittin, contains tryptophan, but no histidine, tyrosine, or methionine residues, it has been selected for further studies to identify the products derived from AAPH-mediated oxidation of tryptophan. Contrary to the reports by other workers using other kinds of reactive oxygen species, N-formylkynurenine and kynurenine could not be detected among the products formed by AAPH oxidation. Positive identification of the products formed by each kind of reactive oxygen species is of interest because they may lead to the development of specific diagnostic tests to determine what kind of oxidative damage is of primary importance in the etiology and/or progression of various pathologies and aging.